U.S. patent number 4,487,213 [Application Number 06/416,163] was granted by the patent office on 1984-12-11 for mass flow controller apparatus.
This patent grant is currently assigned to Omicron Technology Corporation. Invention is credited to William E. Gates, Wim J. M. Pieters.
United States Patent |
4,487,213 |
Gates , et al. |
December 11, 1984 |
Mass flow controller apparatus
Abstract
An apparatus capable of selecting and controlling the mass flow
rate of a gas to be substantially constant over extended periods of
time at very low flow rates and very low gas partial pressures is
disclosed.
Inventors: |
Gates; William E. (Sommerset,
NJ), Pieters; Wim J. M. (Morristown, NJ) |
Assignee: |
Omicron Technology Corporation
(Berkeley Heights, NJ)
|
Family
ID: |
23648822 |
Appl.
No.: |
06/416,163 |
Filed: |
September 9, 1982 |
Current U.S.
Class: |
137/2; 137/486;
137/487.5; 73/204.22 |
Current CPC
Class: |
G01F
1/6847 (20130101); Y10T 137/0324 (20150401); Y10T
137/7761 (20150401); Y10T 137/7759 (20150401) |
Current International
Class: |
G01F
1/684 (20060101); G01F 001/68 () |
Field of
Search: |
;137/486,487.5,2,3,4
;73/204,861.04 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Chambers; A. Michael
Assistant Examiner: Malkin; Mark
Claims
What is claimed is:
1. An apparatus capable of selecting and controlling the mass flow
rate of gas through a conduit to be substantially constant over a
period of at least 4 hours and within a mass flow rate range of
from about 0.05 to about 0.5 ml/min at standard temperature and
pressure conditions and a gas partial pressure range of from about
0.03 to 1 which comprises:
an enclosure having an exterior surface and defining an inner
space;
means for controlling the temperature within said inner space to be
substantially constant;
first conduit means defining an input path for directing the flow
of a gas from an input port disposed at the exterior surface of
said enclosure into said inner space, and an output path in fluid
communication with said input path for directing the flow of said
gas out of said inner space to an output port at the exterior
surface of said enclosure;
elongate sensing conduit means disposed within said enclosure inner
space having an interior channel for directing the flow of a gas
therethrough and having an input and an output; the input of said
sensing conduit being engaged in fluid communication with the input
path of said first conduit means and the output of said sensing
conduit being engaged in fluid communication with the output path
of said first conduit means, the diameter of said sensing conduit
interior channel being not greater than 0.2 mm;
flow meter means disposed within said enclosure inner space and in
contact with said sensing conduit for sensing a temperature
differential induced by change in the mass flow rate of a gas
passing from the input to the output of said sensing conduit, and
for generating a first output signal representative of said
temperature differential and proportional to the mass flow rate of
said gas passing through said sensing conduit;
means for generating a reference second output signal proportional
to a selected mass flow rate of a gas;
means disposed within said enclosure inner space for comparing the
first and second output signals and generating a third output
signal representative of the difference of said first and second
output signals;
valve means, disposed within said enclosure inner space engaged
with said sensing conduit downstream of said flow meter means and
with the output path of said first conduit means for controllably
disengaging the fluid communication between the sensing conduit and
the output path of said first conduit means thereby permitting
regulation of the mass flow rate of a gas passing from said sensing
conduit to the output path of said first conduit means, said valve
means being actuated by said third output signal;
means for permitting the temperature of a gas passing through the
input path of said first conduit means to equilibrate with the
substantially constant temperature of said enclosure inner space
before said gas enters the sensing conduit means;
means for controlling the pressure of a gas as it enters the input
of said sensing conduit means to be substantially constant.
2. The apparatus of claim 1 wherein said flow meter means
comprises:
a plurality of self-heating sensor element coils having adjacent
ends positioned along the flow path of a gas flowing through said
sensing conduit means externally of said sensing conduit means, one
of said sensor elements being closer to one end of said sensing
conduit means than another of said sensor elements, said sensor
element coils being formed of temperature sensitive resistant wire
wound around the outer surface of said sensing conduit means and
sensing their own temperature as modified by changes in the mass
flow rate of said gas;
means for heating said sensor elements;
means for detecting the temperature differential of said sensor
elements induced by said mass flow rate change; and
means for generating said first output signal.
3. The apparatus of claim 1 wherein the diameter of the interior
channel of the sensing conduit is not greater than 0.05 mm.
4. The apparatus of claim 1 wherein said means for controlling and
selecting the temperature of said enclosure inner space
comprises:
vent means disposed within said enclosure for engaging said inner
space in fluid communication with the atmosphere exterior to said
enclosure, thereby permitting the enclosure inner space to receive
air from or discharge air to the environment external to said
enclosure;
means for sensing the temperature of the atmosphere within said
enclosure inner space and for generating a first temperature output
signal proportional to said temperature;
means for generating a second temperature output signal
proportional to the difference between said first output signal and
a reference signal representative of a selected temperature;
means for heating the atmosphere within said inner space actuated
by said second output signal; and
means for continuously circulating air from external to said
enclosure uniformly within said inner space at a rate of from about
10 to about 75 times the volume of said inner space per minute.
5. The apparatus of claim 1 wherein the input path of said first
conduit means is adapted to form a coil for permitting the
temperature of gas passing therethrough to equilibrate with the
temperature of said inner space.
6. The apparatus of claim 1 wherein said valve means is a thermal
valve responsive to thermoexpansion of an actuator relative to a
reference number.
Description
BACKGROUND OF THE INVENTION
The present invention relates to an improved mass flow controller
apparatus which is capable of controlling the mass flow rates of a
gas through a conduit to be substantially constant at very low mass
flow rates, and very low gas pressures.
A conventional mass flow controller is an electronic device which
operates on an adjustable aperature principle and is designed to
deliver a fluid such as a gas or liquid through a conduit at a
preselected mass flow rate.
The primary components of a typical commercial mass flow controller
include a mass flow meter which continuously senses the actual flow
rate of a gas and generates an electrical signal proportional to
the mass flow rate; a command signal generator which provides an
electrical signal proportional to a preselected mass flow rate; a
comparator controller which compares the flow meter signal with the
command signal, takes the difference between the two signals and
uses the difference, if any, to actuate an electrically responsive
valve, the valve also constituting a component of the flow
controller.
In conventional mass flow meters, heat is applied to a sensing tube
conducting the fluid to be measured or is directly applied to a
fluid and the temperature of the fluid is measured after the
addition. When the upstream temperature is equal to the unheated
stream temperature, mass flow can be measured as inversely
proportional to the temperature difference for a constant heat
addition. A conventional bridge circuit can be used to obtain an
electrical signal versus flow function. In another arrangement,
heat is applied to a sensing tube and the temperature of the tube
measured before and after the heat addition. The upstream
temperature of the fluid is influenced by the heating of the tube
and is nearly equal to the heater temperature at zero flow. The
mass flow of the fluid is proportional to the temperature
differential for a constant heat addition. In a third arrangement,
heat is applied to a very small wire, probe or thermistor in the
fluid stream and the cooling effect of the fluid stream is
measured. Cooling of the element is a function of the mass flow. In
still another arrangement, heat is applied uniformly to a tube by
resistance heating and the cooling effect of the fluid measured
with thermocouples to determine mass flow. In yet another
arrangement, a pair of temperature sensitive resistance wire coils
are wound around the outer surface of a sensing tube through which
the fluid flows. The coils are heated and the rate of mass flow of
the fluid, which is directly proportional to the temperature
differential of the coils, is measured by a bridge circuit.
All of the above arrangements to some extent rely on the generation
of a temperature differential induced by a change in the flow rate,
e.g. by differential cooling or heating of the fluid, as an
indication of the mass flow rate of the fluid. This necessarily
requires that the fluid be thermally conductive.
The last sensing arrangement described above, i.e. using a pair of
heated sensor coils, is described in Blair, U.S. Pat. No.
3,938,384, and provides increased sensor efficiency due to reduced
loss of heat in the sensor elements under conventional operating
conditions. In an alternative embodiment disclosed in this patent
at FIG. 2 a flow splitting technique is employed to permit
measurement of high flow rates. The flow splitting technique
employs a bypass tube, which diverts a very small percentage of the
fluid in the main tube to flow therein. In accordance with
conventional flow splitting techniques, as described in more detail
in U.S. Pat. No. 3,851,526, the bypass tube is typically a very
thin tubular conduit which is much longer than its diameter to
assure laminar flow therein. Consequently, it is not surprising
that the bypass tube constituting the sensing conduit of FIG. 2 in
the Blair patent is of capillary dimensions (i.e. 0.014 inches at a
length of 3.0 inches). The diameter of the main flow conduit 52 of
FIG. 2 is not disclosed nor is the diameter of sensing conduit 14
of FIG. 1 therein. Blair also discloses the use of an insulator
material of low thermal conductivity, form-fitted around the
sensing elements of the sensing conduit. The insulator material
reduces the attitude sensitivity of the flow meter. In a further
alternative embodiment, Blair suggests, at col. 5, the use of
either two constant current sources to replace the bridge resistors
of the flow meter bridge circuit, or the use of two bridge
resistors of very high resistance relative to the resistance of the
sensing elements coupled with a single constant current source in
series with the flow meter voltage source, to reduce the influence
of ambient temperature changes on the accuracy of the flow
meter.
A conventional electrically operated valve is a thermal valve
disclosed in U.S. Pat. No. 3,650,505 the disclosure of which is
herein incorporated by reference. The thermal valve utilizes heat,
generated by an electrical input signal to expand an actuator
relative to a reference member. The actuator and reference member
are interconnected to one end, and one or the other carries a valve
head on its opposite end whereby a differential in expansion caused
by a heating element moves the valve head from its seat. Thus the
command signal generates heat which is used to open the valve by
thermoexpansion. Representative gaseous flow rates typically
handled by such thermal valve as disclosed in this patent range
from 50 to 3,000 cc/min.
The command signal generator, and comparator controller are all
conventional devices.
In commonly assigned U.S. patent application Ser. No. 416,164 filed
on an even date herewith by the inventors herein, the disclosure of
which is herein incorporated by reference, a method and an
analytical device is described for determining the amount of a gas
absorbed or desorbed from a solid sample. To conduct this method
successfully, the capability is required of continuously
controlling the mass flow rate of the gas being administered to, or
withdrawn from, the sample to be substantially constant (as
hereinafter defined) over extended periods of time of from about 4
to about 20 hours. This method further requires that the mass flow
rate be capable of being controlled to be substantially constant
when operating over a partial pressure (P/P.sub.s) range of the gas
of at least between 0.02 to 1, and at very low flow rates of
between about 0.2 to about 0.4 ml/min at standard temperature and
pressure conditions. The partial pressure of a gas, sometimes
referred to as relative pressure, is the pressure (P) of a gas at a
given set of temperature and volume conditions divided by the
pressure (P.sub.s) of the gas at the same conditions at which
liquefaction or saturation thereof occurs. Thus, at a partial
pressure of about 0.04 the gas exists in a state very near a
complete vacuum. Unfortunately, it has been found that conventional
mass flow controllers such as those employing the above described
components are not capable of sustaining, over extended periods of
time, a substantially constant mass flow rate over the entire range
of mass flow rates and partial pressures described above. The need
therefore arose to develop a device which possessed these
capabilities, and the present invention was developed in response
to this need.
SUMMARY OF THE INVENTION
It has been discovered that when employing conventional mass flow
controllers of the type described above, fluctuations in the mass
flow rate of a gas at very low pressures and flow rates can be
attributed to the combination of (a) the low thermal conductivity
of the gas at very low pressures and flow rates, (b) fluctuations
in the temperature of the gas as it enters the mass flow
controller, and (c) fluctuations in the temperature of the
electronic circuitry of the mass flow controller.
More specifically, at very low gas pressures and flow rates in the
sensing conduit of conventional mass flow controllers, the density
of the gas can become sufficiently low that it substantially
reduces the thermal conductivity of the gas. This in turn disturbs
the accuracy of thermal sensing mechanism of such flow meters which
require the ability to detect temperature differentials associated
with changes in the mass flow rate of the gas. Fluctuations in the
temperature of the gas as it enters the sensing conduit of the mass
flow controller and fluctuations in the electronic circuitry
temperature are caused by fluctuations in ambient room temperature
conditions. Fluctuations in the gas temperature generate
temperature differentials in the gas which are not proportional to
the mass flow rate thereby leading to inaccuracy in the flow meter
sensing mechanism. Fluctuations in the temperature of electronic
circuitry of the mass flow controller changes the resistance of
various circuit components thereby also contributing to error in
the sensing and feedback mechanisms responsible for mass flow rate
control. In addition, fluctuations in the pressure of the gas are
slowly compensated for by conventional mass flow controllers, and
also contribute to the inaccuracy of the flow rate.
While each of the aforenoted fluctuations and associated error
caused thereby in the mass flow rate may be small over very short
periods, they accumulate with time, and in the span of 4 to 20
hours or more they contribute to a sufficiently large deviation in
the mass flow rate over such periods that they are not
substantially constant as defined herein.
The aforenoted problems are solved by the apparatus of the present
invention by employing a capillary tube as the sensing conduit of
the flow meter to increase the density of the gas flowing
therethrough, and by providing a temperature controlled environment
which simultaneously maintains the temperature of the gas entering
the sensing conduit constant as well as the temperature of the
electronic circuitry of the mass flow controller.
Accordingly in accordance with the present invention there is
provided an apparatus capable of selecting and controlling the mass
flow rate of gas through a conduit to be substantially constant
over a period of at least 4 hours and within a mass flow rate range
of from about 0.05 to about 0.5 ml/min at standard temperature and
pressure conditions and a gas partial pressure range of from about
0.03 to 1 which comprises:
an enclosure having an exterior surface and defining an inner
space;
means for controlling the temperature within said inner space to be
substantially constant;
first conduit means defining an input path for directing the flow
of a gas from an input port disposed at the exterior surface of
said enclosure into said inner space, and an output path in fluid
communication with said input path for directing the flow of said
gas out of said inner space to an output port at the exterior
surface of said enclosure;
elongate sensing conduit means disposed within said enclosure inner
space having an interior channel for directing the flow of a gas
therethrough and having an input and an output; the input of said
sensing conduit being engaged in fluid communication with the input
path of said first conduit means and the output of said sensing
conduit being engaged in fluid communication with the output path
of said first conduit means, the diameter of said sensing conduit
interior channel being not greater than 0.2 mm;
flow meter means disposed within said enclosure inner space and in
contact with said sensing conduit for sensing a temperature
differential induced by change in the mass flow rate of a gas
passing from the input to the output of said sensing conduit, and
for generating a first output signal representative of said
temperature differential and proportional to the mass flow rate of
said gas passing through said sensing conduit;
means for generating a reference second output signal proportional
to a selected mass flow rate of a gas;
means disposed within said enclosure inner space for comparing the
first and second output signals and generating a third output
signal representative of the difference of said first and second
output signals;
valve means, disposed within said enclosure inner space engaged
with said sensing conduit downstream of said flow meter means and
with the output path of said first conduit means for controllably
disengaging the fluid communication between the sensing conduit and
the output path of said first conduit means thereby permitting
regulation of the mass flow rate of a gas passing from said sensing
conduit to the output path path of said first conduit means, said
valve means being actuated by said third output signal;
means for permitting the temperature of a gas passing through the
input path of said first conduit means to equilibrate with the
substantially constant temperature of said enclosure inner space
before said gas enters the sensing conduit means; and
means for controlling the pressure of a gas as it enters the input
of said sensing conduit means to be substantially constant.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram of the component parts of the mass
flow controller of the present invention.
FIG. 2 is a more detailed schematic diagram of the mass flow
controller circuit board 401 and flow regulating means within block
403 of FIG. 1.
The diagrammatic showing of FIGS. 1 and 2 omit in certain instances
features which those skilled in the art would recognize as
desirable in actual apparatus operation. These omissions are made
in order to simplify the presentation of the invention and to avoid
encumbering it with well understood engineering details. Thus, for
example, certain equipment obviously needed for a power supply, for
electrical connections to the power supply, etc. are omitted from
the diagrammatic representation.
DESCRIPTION OF PREFERRED EMBODIMENTS
The primary components of the mass flow controller of the present
invention comprise an enclosure, typically in the form a box to
permit substantially constant temperature control of the contents
of the enclosure, and the functional components of the mass flow
controller.
By "substantially constant" as used in connection with the
temperature within said enclosure is meant fluctuations, if any, of
typically not greater than .+-.0.5, preferably not greater than
.+-.0.2, and most preferably not greater than about .+-.0.1.degree.
C. in said temperature. The temperature of the enclosure must be
capable of being maintained substantially constant over extended
continuous periods of at least about 4, preferably at least about
20 hours, e.g. typically the time during which the mass flow
controller will be in continuous operation when employed for the
purposes described in the aforenoted commonly assigned patent
application for which the mass flow controller described herein can
be used.
The term "substantially constant" as applied to the mass flow rate
of a gas being controlled by the mass flow controller is defined
herein to mean fluctuations, if any, of not greater than .+-.0.4%,
preferably not greater than .+-.0.2%, and most preferably not
greater than .+-.0.15% in said mass flow rate when the mass flow
rate is not less than 0.2 ml/min. When the mass flow rate is
between about 0.05 to about 0.19, the term "substantially constant"
as applied thereto is defined herein the mean fluctuations, if any,
of not greater than about .+-.1% of said mass flow rate. As the
mass flow rate decreases below about 0.05 ml/min the fluctuation
therein may increase above 1%.
The mass flow controller described herein is capable of maintaining
the flow rate a gas through a conduit to be substantially constant:
(a) for a continuous period of at least 4, preferably at least 20
hours; (b) over a partial pressure range of said gas within the
sensing conduit described herein of typically from about 0.04 to 1,
preferably 0.03 to 1; and most preferably 0.02 to 1; and (c) over a
mass flow rate range of said gas at standard gas temperature and
pressure conditions of typically from about 0.05 to about 0.7,
preferably from about 0.2 to about 0.5, and most preferably from
about 0.2 to about 0.4 ml/min. As one skilled in the art would
appreciate, the mass flow controller would also be operable under
the less stringent conditions encountered by conventional mass flow
controllers described in the background herein, such as controlling
mass flow rates of 50 ml/min or higher at 1-5 atmospheres of
pressure.
The gas flow rates described herein are characterized as mass flow
rates because the mass flow controller described hereinafter,
responds to thermal conductivity of a gas which is proportional to
the mass of the gas. Therefore milliliters per minute can be
converted to mass by the Arrhenius equation.
A preferred embodiment of the mass flow controller of the present
invention is illustrated collectively in FIGS. 1 and 2 and is best
described with reference to these figures.
Referring to FIG. 1, the components of the mass flow controller are
housed in a temperature controlled box 119. This box encloses an
inner space 608; which is in fluid communication with the
atmosphere outside the box via vent 601 and hence is typically
filled with air. The box is typically made of a sturdy material
such as sheet metal and typically encloses a volume of from about
1500 to about 2500 cm.sup.3.
The temperature of the atmosphere within the box preferably is
maintained above ambient room temperature to permit active control
of the box temperature and thereby maintain such temperature
constant. Typically, this temperature is maintained between about
35.degree. and 45.degree. C. (e.g. 39.degree. C.) to avoid damaging
the electronics housed within the box and to provide a sufficient
temperature gradient between the atmosphere within and without the
box that when room air from outside the box is drawn into the same
by fan 407, a dynamic temperature equilibrium can be established
within the box. Thus, the box temperature selected preferably will
be above the temperature which the box would attain, absent a
separate heat source, due to the heat given off by the electronics
of the mass flow controller. Consequently, when air from the
external environment of the box is mixed with air in the box and
heated as described herein, the temperature equilibrium established
within the box is not disturbed by the mass flow controller
electronics.
Temperature control within the box is achieved by the combination
of thermistor 404 which preferably is attached to the base of flow
controller block 403, Proportional Integral and Derivative
(hereinafter PID Controller 405, heating strip 406, and fan 407.
Thermistor 404 senses the air temperature within the box over a
range of 0.degree. to 100.degree. C. and generates an electrical
signal proportional thereto which is sent to the PID controller.
The controller (e.g., RFL model 70A) is preset to generate an
electrical signal proportional to the difference between a preset
box temperature and the actual box temperature. The PID generated
electrical signal activates a heater strip 406 (e.g., of the
resistive type 300 watt capacity). Thus, the strength of the
electrical signal from the PID controller to the heater gets
progressively weaker as the actual box temperature approaches the
preset temperature and the signal is substantially constant when
the actual box temperature equals the preset temperature to
compensate for the heat loss to the environment. This provides
extremely accurate temperature control. The fan 407 runs
continuously during operation of the mass flow controller and
circulates air in the box around the heating strip at a high rate
of from about 10 to 100 (e.g., 10-75) times the volume of the box
or higher per minute.
The fan 407 preferably is located close to vent 601 to draw fresh
air into the box 119. The primary components of the mass flow
controller which are located within box 119 include (a) stainless
steel block 403 adapted to contain sensing conduit 201a which runs
along the length thereto until it connects with thermal valve 302,
sensing coil 411, and the controllable aperature portion of thermal
valve 302 located downstream of sensing coil 411; (b) circuit board
401 which contains the electronic circuitry of the flow controller
including, detector bridge circuit 408, linearizer 409, amplifier
410, and comparator controller 600; and (c) thermal valve 302.
Gas is introduced and withdrawn from box 119 and sensing conduit
201a via conduit lines 201 and 202 respectively. Conduit line 201
will typically be connected to a feed gas source. Engaged in series
with conduit line 201, is a pressure controller 311 (e.g. SERTA.TM.
model 204) capable of maintaining the pressure of the gas being fed
to sensing Conduit 201a to be substantially constant. By
substantially constant as applied to the incoming gas to sensing
conduit 201a is defined herein to mean fluctuations, if any, in
said pressure of typically not greater than about .+-.0.35,
preferably not greater than about .+-.0.3, and most preferably not
greater than about .+-.0.2 psi. The pressure in conduit 201 will
typically be controlled to be from about 16 to about 25 and
preferably from about 16 to about 18 psi. A portion of conduit line
201, typically downstream of pressure controller 311, is adapted to
form coil 213 wherein the temperature of gas present therein
equilibrates with the temperature of box 119. Coil 213 also assists
in stabilizing the pressure in Conduit 201, due to its higher inner
diameter relative to Conduit 201. Coil volumes of coil 213 of
typically from about 0.5 to about 1.5 cm.sup.3 are sufficient for
this purpose. Conduit lines 201 and 202 typically are made of
stainless steel with an inner diameter of from about 0.05 to about
0.2 inch.
Thus, controlling the temperature of box 119 performs at least 2
important functions, namely, (1) it heats incoming gas in line 201
so that by the time it reaches the flow meter sensing coil 411, it
is at the same constant temperature as the box 119 irrespective of
fluctuations in the room temperature; and (2) it compensates for
the temperature sensitivity of the electronic circuitry present in
the circuit board 401, and thermal valve 302.
Sensing conduit 201a starts at the entrance to block 403, passes
through sensing coil 411 (which is shown as a single coil in FIG. 1
but is more specifically shown in FIG. 2 as two coils 411a and
411b) and connects with the inlet portion of thermal valve 302. The
outlet portion of thermal valve 302 is connected to line 202 by
which gas exits block 403 and box 119. The body and electronic
circuitry of thermal valve 302 is seated on top of block 403 and
encased in plastic housing 412.
The preferred thermal valve is available from Tylan Corporation and
is described in detail in U.S. Pat. No. 3,650,505, the disclosure
of which is herein incorporated by reference.
Circuit board 401 is encased in plastic housing 402.
The circuitry and operation of the mass flow controller is best
described with reference to FIG. 2. Broken line 516 of FIG. 2
depicts the flow meter portion of the flow controller which
contains bridge circuit 517 coupled to sensing conduit 201a. The
bridge circuit is of conventional design and is formed of a first
bridge resistor 501 and a second bridge resistor 502. The bridge
circuit further comprises an upstream sensor element 411a and a
downstream sensor element 411b. The sensor elements 411a and 411b
are wound around the sensing conduit 201a adjacent each other with
the upstream sensor element 411a closer to input end 518 of tube
201a and the downstream sensor element 411b closer to the output
end 519 of conduit 201a.
The bridge circuit 517 also comprises a D.C. power supply and
converter 506, (which operates from AC power source 507) which is
connected at one side via line 604 between the junction of the
sensor elements 411a and 411b. The other side of the power supply
is connected through a switch 505 to the junction of the bridge
resistor 501 and 502. Output signals from the bridge circuit are
coupled at a first output terminal 503 and a second output terminal
504. The first output terminal 503 is connected to the junction of
the upstream sensor element 411a and first bridge resistor 501, and
the second output terminal 504 is connected to the junction of the
downstream sensor element 411b and the second bridge resistor 502.
The upstream sensor element 411a and the downstream sensor element
411b are formed of temperature-sensitive resistance wire which is
wound around the outer diameter of the conduit 201a. Such wire can
be an iron-nickel alloy, e.g. Balco (a trademark of the
Wilbur-Driver Company).
The above circuit design is conventional with the exception that
the inner diameter of sensing conduit 201a must be of capillary
size of not greater than about 0.2. preferably not greater than
0.05, and most preferably not greater than 0.02 mm, and typically
ranges from about 0.005 to about 0.2, preferably from about 0.01 to
about 0.1, and most preferably from about 0.01 to about 0.05 mm to
achieve substantially constant flow rate. Furthermore, because of
the low pressures and flow rates handled by the flow meter, a
by-pass tube conventionally employed in mass flow meters should be
avoided. As stated above, it has been found that if the inner
diameter of sensing conduit 201a is too large, the density of the
gas within the tube becomes so low at the pressures and flow rates
described herein that the thermal conductivity of the gas drops to
the point where the thermal sensing mechanism (described
hereinafter) of the flow meter is disrupted and substantially
constant flow rate is not attained.
The coupled bridge circuit output signals 503 and 504 are connected
to linearizer 409 which electronically provides a linear output
voltage as a function of mass flow. This voltage is applied through
amplifier 410 to comparator controller 600.
In operation, when the switch 505 is closed, current flows through
sensor elements 411a and 411b causing the sensor elements to
generate heat, thereby raising the temperature of the tube 201a
adjacent the elements. Heating of elements 411a and 411b also
raises their resistance. At zero fluid flow through the tube 201a,
the temperatures of the sensor elements 411a and 411b are equal and
the bridge is therefore balanced, producing a zero output voltage
across the terminals 503 and 504. As fluid enters the input end 518
of the tube 201a, heat generated by the elements 411a and 411b is
carried by the fluid downstream toward the output end 519 of the
tube 201a. Thus, a temperature differential is created between the
elements 411a and 411b due to the shifting temperature profile
along the tube 201a. As the flow of fluid increases in the tube
201a, the temperature of the upstream element 411a as well as its
resistance decreases while simultaneously the temperature of the
downstream element 411b, as well as its resistance, increases. The
bridge output voltage at terminals 503 and 504 thereby increases in
nearly linear proportion to the flow rate. After linearization and
amplification of the bridge output voltages of terminals 503 and
504, amplifier 410 applies a single linear voltage which
corresponds to the absolute value of the mass flow rate of the gas
before the gas reaches thermal valve 302. The voltage from
amplifier 410 is applied to comparator controller 600 where it is
compared to an external electrical command signal 514 from command
signal source and selector 520 which has been preset to correspond
to a selected mass flow rate. The comparator controller establishes
the difference, if any, between the command signal voltage and the
amplifier voltage and utilizes it to power the actuator 302a of
thermal valve 302 via line 603. The thermal valve actuator is thus
controlled so that it opens more when the comparator control
indicates that the mass flow rate is insufficient to balance the
command signal, and is closed more when the opposite situation
occurs.
In a more preferred alternative embodiment sensor elements 411a and
411b can be combined into a single coil containing a center tap as
illustrated in U.S. Pat. No. 3,938,384, the disclosure of which is
herein incorporated by reference. By utilizing a single coil with a
center tap rather than two separate sensor elements, it is possible
to space the coils close together. Thus, heat loss is reduced,
equalization between the upstream and downstream sensor elements is
facilitated, and the gain of the circuit (temperature change per
unit of flow) is greater. In addition, the response of the circuit
is faster, and the range of useful flow measurement and the
linearity of the circuit is increased.
It is to be understood, that while the mass flow controller has
been described with respect to a particular flow meter, other types
of flow meters can be used which provide an electrical signal vs.
flow function and which operate with a thermoresponsive sensing
mechanism, such as described in U.S. Pat. Nos. 3,650,151,
4,056,975; 4,100,801 and the like as described in the background
herein. While a thermal valve as described herein is preferred to
regulate the flow rate, any other electrically actuated means which
possesses the sensitivity and capability of regulating the flow
rate to be substantially constant at the conditions of use
described herein can be employed.
Furthermore, it is pointed out that the insulating foam block
encasing sensor elements as described in U.S. Pat. No. 3,938,384,
is preferably avoided because it disturbs the temperature
equilibration rate of sensing elements 411a and 411b with the box
119 temperature and is not needed.
The principles, preferred embodiments, and modes of operation of
the present invention have been described in the foregoing
specification. The invention which is intended to be protected
herein, however, is not to be construed as limited to the
particular forms disclosed, since these are to be regarded as
illustrative rather than restrictive. Variations and changes may be
made by those skilled in the art without departing from the spirit
of the invention.
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